Optical position devices are well known. They are utilized in a variety of applications, including common automatic toilet flushers, video games, and sophisticated laser guided munitions. FIG. 1 is a simplified depiction of a laser guided munitions application. As shown, a laser beam 110 is aimed at a building 115 from an airplane 105. A missile 100 includes a seeker 120 that senses laser light 125 reflected from the building 115. The seeker 120 tracks the reflected laser light 125, guiding the missile to the building 115.
There are two primary methods of optical position detection. One utilizes a Position Sensitive Detector (PSD), and the other utilizes a Quad Detector. Both a PSD and a Quad Detector include a lens to focus light onto a detector surface.
FIG. 2A is a first depiction of a PSD showing a lens 201 focusing light 205 onto a detector surface 210. Processing electronics 215 convert angular displacement into linear displacement. A PSD gives lateral position of a focused image of a light source by generating a differential current. As shown in FIG. 2B, one cathode (C) and at least two anodes (A) are in contact with a detector surface 210. Current through an anode is proportional to the position of the light imaged onto the detector surface 210 due to resistance of the detector surface 210. In the example of FIG. 2B, focused light 205 strikes the detector surface 210 two-thirds of the width of the detector surface 210 away from anode A2, and one-third the width of the detector surface 210 away from anode A1. As a result of the resistance, one-third of the produced current flows through anode A2, and two-thirds of the produced current flows through anode A1. The current from the two anodes is then processed by the processing electronics 215 to determine where on the detector surface 210 the focused light 205 is imaged to determine an angular position of the light source.
The advantage of utilizing a PSD is that continuous angular position can be determined throughout the PSD's angular field of view. However, disadvantages associated with PSD's include a slow response time, typically in the range of milliseconds, which is too slow for many applications, and a high noise floor, resulting in low sensitivity. Thus, PSD's cannot be used in certain applications, such as laser guided munitions. PSD's are typically relegated to more common applications, such as automatic toilet flushers and automatic sinks.
FIG. 2C is a simplified depiction of a Quad Detector. A Quad Detector includes four detector surfaces 220A–D constructed on a single piece of silicon. Similar to a PSD, a lens 201 focuses light 205. The light 205 is focused on the junction where the four detectors 220A–D converge, and, when properly aligned, falls equally on all four detectors 220A–D. If the junction is not properly aligned, the light 205 will fall disproportionally onto the four detectors 220A–D, causing unequal signal strength from the four detectors 220A–D. The lens 201 must be mechanically realigned to properly position the light 205 on the junction of the four detectors 220A–D. This mechanical realignment is achieved utilizing a gimbal (not shown in the Figures). Detected light 205 is then sent to processing electronics 215 to determine the angular position of the light source. An advantage of utilizing a Quad Detector is that Quad Detectors have a fast response time. Thus, they are utilized in sophisticated applications, such as laser guided munitions. A disadvantage is that an expensive gimbal must be utilized for continuous angular position tracking within the Quad Detector's field of view.
Accordingly, a need exists for an optical position device that has a fast response time and does not require the use of a gimbal.
Both PSD's and Quad Detectors also suffer from a waste of detector sensitivity. While a good lens may have a F# of F2, and an excellent lens may have a F# of F1, most of the sensitivity provided by lenses in optical position devices is wasted. FIG. 3 is a simplified depiction the wasted sensitivity. The angular sensitivity subtended by a typical optical detector such as a PSD or Quad Detector is pi. However, a lens uses only a small fraction of this subtended sensitivity. For example, an excellent F1 lens only subtends 0.58 pi. This excellent F1 lens wastes greater than 70% of the available pi sensitivity of the detector.
Accordingly, a need exits for an optical position device which does not utilize a lens.